Transformer including boron nitride insulation



Feb. 18, 1969 R. Q. MAINES 3,428,923

v TRANSFORMER INCLUDING BQRON NITRIDE INSULATION Filed Nov. 18, 1966 Sheet of 2 FIG.3

INVENTOR. ROBERT 6 MA INES A T TORNEKS R. Q. MAINES Feb.'l8, 1969 TRANSFORMER INCLUDING BORON NITRIDE INSULATION Filed Nov. 18, 1966 Sheet INVENTOR. P0552? Q-MA/IVES' li'yll I V I J FIG.4

I w I F'IGIS MW United States Patent I 3,428,928 TRANSFORMER INCLUDING BORON NITRIDE INSULATION Robert Q. Maines, Wilton, Conn., assignor to Ovitron Conn, a corporation of ABSTRACT OF THE DISCLOSURE A transformer includes boron nitride sheets interspersed between adjacent winding layers and extending longitudinally to the housing to provide a medium for the longitudinal transfer of heat.

This is a continuation-in-part of my application, Ser. No. 402,400, filed Oct. 8, 1964, entitled Power Transformer, now abandoned.

- In small transformers the surface area is relatively large compared to the overall volume, and hence cooling by radiation and natural air convection are generally sufiicient to keep the operating temperature below that which would otherwise deteriorate the winding insulation. As the size and k.v.a. rating of the transformer increases, however, the surface area increases at a slower rate than that of the volume and some means must be employed to' accelerate theheat dissipation.

In many cases paper or some equivalent organic material, frequently impregnated with a liquid such as oil, have been employed in transformers for insulation and for heat dissipation. Oil is a far better heat dissipator than the treated paper compound and has therefore been used for higher power transformers. However, even where oil is practical, several of its inherent characteristics require compensation and safeguards. For example, heating of the core leads to evaporation loss of the oil; moisture drastically reduces oils efficiency; thermal expansion of oil stresses the casing; the low-flash point of oil creates a fire hazard (which precludes its use in many areas); poor thermal conductivity of oil requires its circulation (natural or forced); and its liquidity requires careful and hermetic sealing of all joints to prevent the exit of the oil and the entrance of moisture.

, Accordingly, it is an object of this invention to devise transformers which exhibit few, if any, of the disadvantages of transformers employing treated paper compounds or oil-filled structures.

-It is a further object of this invention to improve the efiiciency and/ or volumetric requirements of transformers generally regardless of their size or power-handling capacity.

In accordance with this invention, boron nitride (BN) in sheet or film form is employed as a spacer between the current-carrying coils of transformers; and, as will be pointed out more fully later, the length of the sheet or film is made so large that it will also provide good thermally-conductive contact with the transformer casing or housing. So designed, it will convey coil-generated heat to the casing or housing to dissipate the heat. BN material has exceptional properties for the above-noted function in that it exhibits, among other properties, relatively high thermal conductivity, which is superior to that of oil and treated paper compounds, and has relatively high dielectric strength, comparable to, and indeed better than, the best of oil and paper compounds.

In carrying out this invention, BN in sheet or film form 3,428,928 Patented Feb. 18, 1969 of sufficient length may be inserted between adjacent coils of wire to freely dissipate heat generation and accumulation and, in order to expand further the dissipation of heat generated by the coils, the BN material will be caused to make direct contact with the transformer casing which itself is usually a good heat conductor and radiator. The BN material will soak up the heat generated by the flow of current through the coils and freely conduct the heat from the coils to the casing (and to its fins). The dissipation will be highly effective even against heat produced by the lower-most layers of coilswhich in conventional structures retain the highest temperatures.

As an outstanding feature in the use of sheets of BN conductively connected to the casing, this invention will involve a transfer of heat from the coil heaters to the casing not only radially in the conventional manner (from coil layer to coil layer) but much more importantly, the heat will also be transferred axially, i.e., it will be channelized along the sheet materials to the casing proper. The longitudinal transfer along the BN material will be much greater and more effective than the radial heat transfer normally encountered but the radial transfer will itself be enhanced by the use of BN, and will result in a sharp increase in the overall etficiency of the transformer.

In order to briefly explain the basic elements of the invention, it may be applied, for simplicity of explanation, to cylindrical paramagnetic cores as follows: a first sheet of BN material, say BN material woven into a tubular shape or otherwise formed into a cylindrical tube, will be slid over the outer surface of the core; then a first layer of windings (i.e., a coil of flat or circular wires) will be wound helically immediately adjacent to the first BN sheet; then a second sheet of tubular BN material, of somewhat larger diameter, will be slid over the first coil layer, then a second coil of wire will be wound about the second BN sheet; and so on for as many layers of sheets and coils as desired. The sheets may have any desired thicknesses to meet the design requirements of the construction. The BN sheets, so formed into concentric tubes, preferably will be made sufliciently longer than the overall length of the coils so that the overhanging portions of the sheets may be brought into direct physical contact with each other and with the casing. This contact will enable generated heat to travel axially, i.e., along the BN sheets, relatively rapidly, to be dissipated by the casing.

The above mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates schematically a perspective of a portion of a transformer which may constitute one embodiment of this invention as applied to a core construction which is of a square or rectangular cross-sectional shape;

FIG. 2 illustrates a broken perspective of the core construction when used with coil windings and BN sheet materials in the practice of the invention;

FIG. 3 shows a top cross-sectional view of the con struction of one form of the transformer arrangement taken along line 33 of FIG. 2;

FIG. 4 shows the .shape of a form of the BN sheet material which may be used in this invention; and

FIG. 5 shows a perspective of one form of arrangement for providing good thermal contact between sheet BN materials and the casing. The figures are merely illustrative and are not drawn to scale. Like reference characters are used throughout the drawing to represent like parts.

Referring to FIG. 1 of the drawing, there may be seen a three-legged iron core construction, shaped in the form r of a figure 8, which may be used in this invention.

The FIG. 1 construction shows the central leg of the core arranged with one sheet of BN material labeled S which is wrapped circumferentially around the outer surface of the central core; a first coil W helically wound about the first BN sheet S and therefore around the central core CO; a second sheet of BN material designated S which is positioned around the first coil W and therefore also wrapped around the common core CO; a second coil W helically wound above and about the second BN sheet S and so on. As will be apparent, additional layers of coils and sheets (not shown) may be surmounted one upon another over the common core, according to this invention, so as to provides a sufiicient number of turns to meet the design requirements of the transformer. The construction just referred to will be enclosed in an appropriately shaped casing K (shown in FIG. which is preferably made of good heat-conducting material, and the casing K may also include a plurality of fins F to increase the heat dissipating ability of the apparatus.

It will be apparent that the coil W may be the primary winding of the transformer and A and A its terminals, and the coil W its secondary winding and B and B its terminals. Many more such coils are contemplated for typical transformer constructions. However, if there were a plurality of coils included in the construction, some of these, preferably the innermost coils of a predetermined number, would be interconnected with each other to constitute the primary winding while the remaining coils would be interconnected with each other to constitute the secondary winding, as will be obvious to those skilled in the art.

As already suggested in regard to the construction such as that shown in FIG. 1 employing a plurality of currentcarrying coils, the generated heat must be dissipated for the eificient operation of the transformer. This is accomplished in this invention by the employment of the sheets of BN material S S S etc., which are so designed and oriented as to fairly directly dispose of the heat. These sheets are good thermal conductors and also exhibit excellent dielectric characteristics as already explained, and they serve to speedily conduct heat generated by the adjacent coils along the lengths of the respective BN sheets to the common casing K to which they are intimately connected. Hence the casing K and its fins F of FIG. 5 will serve as heat sinks to radiate heat supplied to them longitudinally along the several BN sheets. Any type of construction may be employed to establish good thermal contact between the sheet materials and the casing.

The BN sheets are preferably elongated so that they are much longer than the overall length of the core construction, and it is this surplus of sheet material that serves to make physical contact with the inner surface of the casing K, as shown in FIG. 5. The free ends of the .sheets preferably may be bunched or tied together so as to increase the overall thickness of the several heat conductors at the exit to the sink. Thus, they jointly cooperate to increase conduction of heat away from the coils.

The wires composing the coils may be of any desired cross-section, circular or square or otherwise, and the wires may be ordinary insulated wires of any well-known type. On the other hand, the wires may be bare copper or aluminum or other conductors, in which case insulation for the wires would be provided in order to minimize short circuiting potentialities. Such insulation may be effected by sputtering boron nitride in whatever form over the outer surfaces of the wires. As another alternative, boron nitride powder composed, for example, of boron nitride particles of 5 to 100 microns, may be inserted into the voids between the core structure and the lowermost coil W and between each boron nitride sheet, such as S and the adjacent coils, W and W etc. Boron nitride powder or wire sputtered with boron nitride, when used along with boron nitride sheets, would considerably enhance the thermal distribution properties of the transformer construction.

FIG. 2 is furnished merely to show a perspective of part of the inner laminated core construction CO, the coils W W and the BN sheets 8;, S S etc. The sheets 5;, S S etc., although separated from each other, may be joined together for a considerable distance at or near their free ends (see FIG. 5) to increase their heat carrying capacity so that they may transfer the heat rather freely to the casing K and there dissipated. The boron nitride sheets may be thin layers or films of 5 to 15 microns thick, or indeed they may be sheets of considerably greater thicknesses, up to at least one-quarter of an inch.

FIG. 3 shows a partial cross-sectional view of the laminated core and related construction for a square type of core, showing the BN sheets S and S butted and the coils W and W It will be observed that all of the components have a common center.

FIG. 4 shows a preferred layout of a sheet of BN material which may be used in the above-noted constructions for a core arrangement which is square or rectangular in cross-section. The spacings w may be approximately equal to the overall width of the core or coil over which the sheet is to be applied. The widths W2 may be somewhat longer than the width W1 if the sheet is to be lapped. The length 1 of the central segment of the sheet may approximately equal the overall length of the coil construction. The lengths l and 1 should be sufficient to enable the ends of the sheets to be fastened together if desired and then joined to the inner surface of the transformer casing to provide good thermal contact therewith. Naturally, the widths w would be shorter as shown if a butt connection is desired for the sheets.

FIG. 5 shows a perspective of the general transformer combination. It will be observed that the elongated strips of the various sheets, such as S S etc., are arranged in physical contact with each other and together are brought into intimate physical contact with the inner surface of the outer casing K. The casing K and its fins F provide the heat sink.

The high thermal conductivity of a BN sheet as above described, its high dielectric strength and also its low thermal coeflicient of expansion render this material especially suitable for efficient transformer construction. These properties combine to make available a transformers which is much smaller than conventional transformers of the same or even substantially higher power rating. A transformer constructed in accordance with this invention may be overloaded to a high degree without undergoing decomposition. Its weight is relatively light and, when manufactured even in small quantities, the cost of manufacture will be low. Significantly, it completely avoids any organic or other readily decomposable materials. Hermetical sealing becomes unnecessary. The above-noted properties are suitable for all ranges of transformers no matter what the rated power, but are especially valuable in higher power ratings.

While pure BN is a practical material to carry out this invention, the material may be manufactured with liquid or solid additives, if desired, to enhance the several advantageous properties without degrading effects. The BN material may have a very high purity, such as to percent pure BN material.

While this invention has been shown and described with reference to a transformer, the general principles of this invention may equally be applied to any other forms of electromagnetic structures, especially coils which generate heat and including loading coils.

The sheets of BN may be made in a polymer form, that is, similar to the forms of Teflon. Such materials would have maximum densities. Such a material yields a very high degree of purity and renders such materials especially suitable for this invention. On the other hand, woven sheets may be made with varying densities, that is, loose or tight weaves. Such woven sheets, lacking the higher densities, would have lower thermal conductivity as well as reduced dielectric strength. A transformer utilizing low density sheets might preferably be evacuated to minimize breakdown possibilities.

While this invention has been shown and described in certain particular embodiments merely for the purpose of illustration, it will be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims. For example, while one type of heat sink arrangement has been shown and described, it would also be possible to group the sheet ends and lead them to radiating towers appended to the conventional cylindrical construction. Further, the casing may abut the core portion of the transformer, i.e., the center of the outermost sheet of BN, so that the central portion of the sheet will be in direct physical contact with the casing. In that way, good thermal dissipation will also be provided from the central portions of the core, i.e., in a radial direction (independently of the elongated terminal portions). Hence radial thermal emission from sheet to sheet will also be dissipated by the casing.

What I claim is:

1. A transformer comprising: a core; a pair of windings mounted on said core; an elongated sheet of boron nitride interposed between said windings; and a housing enclosing said core, said windings and said sheet; said sheet being fastened to said housing to provide a medium for the longitudinal transfer of heat from said windings to said housing.

2. A transformer according to claim 1 in which a second elongated sheet of boron nitride is interposed between said windings and said core and is also fastened to said housing to provide a longitudinal heat transfer medium.

3. A transformer according to claim 1 in which powdered boron nitride is filled into the spaces between the windings and the sheet.

4. A transformer according to claim 1 in which boron nitride material is sputtered on the surfaces of said windings.

5. A transformer comprising: a core; a plurality of coils of wire some forming a primary winding and the others a secondary winding; a plurality of elongated sheets of boron nitride each of a length which exceeds the length of the coils, each sheet being inserted between two adjacent coils; a casing enclosing said transformer and having a good conductive connection to all sheets of boron nitride; whereby heat generated in said coils will be transferred longitudinally to said casing and dissipated.

6. A transformer according to claim 5 in which boron nitride powder is filled into the spaces betwen said coils, said core and said casing.

7. In a transformer of the type including at least one multilayer coil wound upon a core of paramagnetic material and an external heat sink for carrying away coil and core heat, the improvement comprising: electrically insulating thermally conducting inorganic sheets interposed in the coil layers and separating them one from another, said sheets extending from said coil layers and being thermally coupled to said heat sink.

8. The transformer claimed in claim 7 wherein said electrically insulating thermally conducting inorganic sheets comprising boron nitride.

9. In an electromagnetic structure including a coil wound upon a core of paramagnetic material and an external heat sink for carrying away coil and core heat, the improvement comprising: electrically insulating thermally conducting inorganic sheets, one of which separates the coil from the core and another of which separates the coil from the sink, said sheets being thermally coupled to said heat sink.

10. An electromagnetic structure according to claim 9 in which the sheets therein recited are composed of boron nitride over 90% pure.

11. An electromagnetic structure according to claim 9 in which the sheets transfer heat both radially and longitudinally to the heat sink.

References Cited UNITED STATES PATENTS 981,690 1/1911 Rhodes 336-61 1,935,885 11/1933 Meissner 336- 2,316,370 4/1943 Smith et a1. 336206 XR 2,354,159 7/1944 Venable 336206 XR 2,770,785 11/1956 Haagens et al. 336-55 XR FOREIGN PATENTS 357,510 1931 Great Britain.

812,162 4/1959 Great Britain.

648,697 1/1951 Great Britain.

OTHER REFERENCES Electronic Design, Sept. 27, 1963, vol. 11, No. 20, p. 47 relied upon.

LEWIS H. MYERS, Primary Examiner. THOMAS J. KOZMA, Assistant Examiner.

US. Cl. X.R. 

